The following abbreviations are herewith defined.
BSC Base Station Controller
BTS Base Transceiver Station
CN Core Network
CRS Cell Re-Selection
DL Down Link (to the MS)
EDGE Enhanced Data rate for Global Evolution
EGPRS Enhanced General Packet Radio Service
GERAN GSM/EDGE Radio Access Network
GGSN Gateway GPRS Support Node
GPRS General Packet Radio Service
GMM GPRS Mobility Management
GSM Global System for Mobile Communications
GSN GPRS Support Node
HO Handover
LLC Logical Link Control
MAC Medium Access Control
MS Mobile Station
MSC Mobile Switching Center
PDP Packet Data Protocol
PDU Packet Data Unit
RLC Radio Link Control
RNC Radio Network Controller
SAPI Service Access Point Indicator
SGSN Serving GPRS Support Node
TBF Temporary Block Flow
TLLI Temporary Logical Link Identity
UL Uplink (from the MS)
UMTS Universal Mobile Telecommunications System
URA User (or UTRAN) Registration Area
UTRAN Universal Terrestrial Radio Access Network
Reference can also be made to 3GPP TR 21.905, V4.4.0 (2001-10), Third Generation Partnership Project; Technical Specification Group Services and System Aspects; Vocabulary for 3GPP Specifications (Release 4), as well as to ETSI TR 101 748, V8.0.0 (2000-05), Digital cellular telecommunications system (Phase 2+); Abbreviations and acronyms (GSM 01.04 version 8.0.0 release 1999).
In general, when the MS (Mobile Station) changes from one cell to another in the GPRS (General Packet Radio Service) system any ongoing TBFs in the old cell are terminated and the new cell is entered. The cell change decision can be made by the MS or by the wireless network. In the case where the network makes the cell change decision, the network sends a Packet Cell Change Order message to the MS. The MS leaves the old cell and enters the new cell almost as if the MS had itself made the cell change decision.
In GPRS there is no handover as in the GSM system, meaning that the radio connections (TBFs) are not maintained during the cell change. When the MS enters the new cell any ongoing TBFs in the old cell are released and re-established in the new cell. A handover procedure in which radio resources are pre-allocated in the new cell is not currently defined in the GPRS specification.
When the GPRS MS is operating in a GMM (GPRS Mobility Management) READY state, and when the MS enters the new cell, the MS is expected to send an LLC PDU (Logical Link Control Packet Data Unit) to the SGSN (Serving GPRS Support Node). Based on the LLC PDU the SGSN is able to determine in which cell the MS is located. As such, the SGSN is not required to initiate a Paging procedure when sending LLC PDUs to the MS.
In the GPRS system, when a MS operating in the GMM READY state changes to a new cell (either MS initiated or network controlled) that is associated with the same routing area as the previous cell, the following process is executed:
(i) the MS determines that it will change to the new cell;
(ii) the MS terminates all TBFs (Temporary Block Flows) in the previous cell, meaning that an ongoing data transfer is terminated abnormally (a TBF is a unidirectional radio connection between MS and the network, where there may be an UL (Uplink) and/or a DL (Downlink) TBF established at any given time);
(iii) the MS enters the new cell;
(iv) the GMM or LLC of the MS (depending on the implementation) requests the LLC to send a LLC PDU to the network;
(v) the LLC of the MS sends the LLC PDU to the RLC/MAC of the MS;
(vi) when the RLC/MAC of the MS has obtained the parameters related to packet access from a System Information message being broadcast in the new cell, or has obtained these parameters via some other means (such as from the previous cell), the RLC/MAC of the MS initiates an UL TBF establishment procedure;
(vii) when the UL TBF is established, the RLC/MAC sends the LLC PDU to the network; and
(viii) the SGSN determines from the received LLC PDU the new location of the MS.
A publication entitled Digital Cellular Telecommunications System (Phase 2+); General Packet Radio Service (GPRS); Mobile Station (MS)-Base Station System (BSS) interface; Radio Link Control/Medium Access Control (RLC/MAC) protocol; (GSM 04.60 version 6.8.0 Release 1997) requires that the received (and segmented) LLC PDUs be put into RLC data blocks in the same order as they are received from higher layers. This means that if there was an ongoing data transfer in the previous cell, the RLC of the MS may have several untransmitted LLC PDUs in its transmit buffer that are to be transmitted to the network in the new cell. The above-mentioned cell change LLC PDU that was initiated by the GMM is thus placed in the last available entry of the transmit queue (i.e., behind any untransmitted LLC PDUs). Before the SGSN receives the first LLC PDU from the MS via the new cell, the SGSN sends all DL LLC PDUs to the previous cell. If the LLC is operating in an unacknowledged (UNACK) mode, the DL LLC PDUs destined to be transmitted to the previous cell are discarded because the network can not contact the MS via the previous cell. If the LLC is operating in an acknowledge (ACK) mode, those DL LLC PDUs destined to be transmitted to the previous cell must be retransmitted to the new cell, which takes time and unnecessarily wastes network resources.
Two examples are now provided to further clarify the current state of the prior art.